1. Field of the Invention
The present invention relates to a draining method used in the construction or renovation of stabilized ground, grass-covered ground such as public parks, green parking lots and particularly sports grounds (football, rugby, hockey, golf, horse-riding tracks, etc.). It also relates to a machine for implementing this method.
Sports grounds are subjected to particular requirements which make their construction very delicate. In the first place, they must withstand intensive and repeated trampling which leads to packing of the earth on the surface and tearing up of the grass. Furthermore, because the maximum use of such grounds takes place in the winter months when bad weather is at its worst, there is a need to provide very good drainage of the water from the surface layer of the ground, so as to prevent the formation of surface mud and an irreversible deterioration phenomenon. However, percolation of water through the soil must be perfectly controlled so that in fine weather the earth retains sufficient humidity for re-growth of the grass. Another important factor to be taken into consideration for the construction of such grounds is their cost price.
2. Description of the Related Art
A conventional technique of constructing sports grounds consists in forming a platform free of topsoil and levelled of the banked up beds between which drains are placed in a herring-bone pattern, forming a layer of pebbles or gravel and covering it with previously improved topsoil. So as to obtain perfect percolation through the ground, it is necessary to add a large amount of river sand to the topsoil, generally in a proportion between 50 and 70%. The large amounts of sand and gravel or pebbles used consequently represent considerable costs and work for laying them. Furthermore, the ground formed using this technique does not give entire satisfaction: in the case of repeated rain, it is difficult to avoid stagnation of the water on the ground so that it becomes practically unusable; and in summer, the presence of the layer of pebbles or gravel results in mediocre water retention of the topsoil, little favorable to a good grass quality: in particular, the rooting depth is not very great.
In a more recent technique, surface drainage trenches are formed in the ground communicating with base trenches equipped with drains. The method is put into practice in the following way: first of all, the layer of topsoil is removed to a depth of about 20 cm: after which the platform is levelled while respecting particular slope conditions natural slope of the ground, transverse slope, longitudinal slope or roof-shaped slope. In practice, the slope is between 1 and 1.5%. Then the base drainage trenches are formed, drains being laid at the bottom thereof and then the trenches are filled with gravel or another filtering material. These trenches are aligned in the direction of the slope, chosen the most often in the transverse direction of the ground. On the side of the ground, the drains communicate with a collector opening into any type of outlet: river, pond, communal network, etc.
The topsoil is then replaced and levelled. The stones are then cleared away mechanically, the soil is decompacted, then the topsoil is improved by spreading basic fertilizers and incorporating sand to a depth of about 10 cm mixing taking place on the site. After checking the inherent flatness, the surface drainage trenches are formed perpendicularly to the base trenches and filled with gravel. They are sufficiently deep for intersecting the base trenches over a height of a few centimeters. Two parameters influence the quality of communication between the surface trenches and the base trenches at their intersection point: on the one hand, the intersection over a height of a few centimeters of the base trench by the surface trench and, on the other hand, the width of the base trench which is chosen very large, generally about 20 cm.
Once the drainage system has been constructed, the whole ground is covered with sand to a thickness of about 2 cm, the flatness is corrected using a beam after which the grass may be sown and rolled.
Different techniques are used for providing surface drainage, one of which consists in forming trenches of a width of about 7 cm with an excavating wheel machine and an earth recovery device, described and claimed in the French patent application 2 546 203 in the name of the present Applicant.
In grounds constructed using the above described technique, drainage is no longer based on percolation through the soil, but on the streaming of water towards the surface drainage trenches, promoted by the slope of the ground. The topsoil therefore does not require a great deal of physical improvement, and the amount of river sand used is reduced, as well as that of gravel or filtering material. On the other hand, the retention of humidity by the topsoil is satisfactory in summer, as well as the rooting depth of the grass obtained.